Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Object-Oriented Design

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Objectives F To become familiar with the process of program development F To the relationship types: association, aggregation, composition, strong inheritance, and weak inheritance F To declare classes to represent the relationships among the classes F To design systems by identifying the classes and discovering the relationships among these classes F To design classes that follow the class-design guidelines F To model dynamic behavior using sequence diagrams and statechart diagrams

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Software Development Process

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Requirement Specification A formal process that seeks to understand the problem and document in detail what the software system needs to do. This phase involves close interaction between users and designers.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved System Analysis Seeks to analyze the business process in terms of data flow, and to identify the system’s input and output. Part of the analysis entails modeling the system’s behavior. The model is intended to capture the essential elements of the system and to define services to the system.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved System Design The process of designing the system’s components. This phase involves the use of many levels of abstraction to decompose the problem into manageable components, identify classes and interfaces, and establish relationships among the classes and interfaces.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Implementation The process of translating the system design into programs. Separate programs are written for each component and put to work together. This phase requires the use of a programming language like Java. The implementation involves coding, testing, and debugging.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Testing Ensures that the code meets the requirements specification and weeds out bugs. An independent team of software engineers not involved in the design and implementation of the project usually conducts such testing.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Deployment Deployment makes the project available for use. This means installing it

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Maintenance Maintenance is concerned with changing and improving the product. A software product must continue to perform and improve in a changing environment. This requires periodic upgrades of the product to fix newly discovered bugs and incorporate changes.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Relationships among Classes F Association F Aggregation F Composition F Inheritance

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Association Association represents a general binary relationship that describes an activity between two classes. An association is usually represented as a data field in the class.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Translation is not Unique NOTE: If you don’t need to know the courses a student takes or a faculty teaches, the data field coureList in Student or Faculty can be omitted.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Association Between Same Class Association may exist between objects of the same class. For example, a person may have a supervisor.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Aggregation and Composition Aggregation is a special form of association, which represents an ownership relationship between two classes. Aggregation models the has-a relationship. If an object is exclusively owned by an aggregated object, the relationship between the object and its aggregated object is referred to as composition.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Representing Aggregation in Classes An aggregation relationship is usually represented as a data field in the aggregated class.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Inner Classes Translation If Name or Address is used in the Person class only, they can be declared as an inner class in Person. For example, public class Person { private Name name; private Address address;... class Name {... } class Address {... }

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Inheritance Inheritance models the is-an-extension-of relationship between two classes.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Weak Inheritance Relationship A weak is-an-extension-of relationship can be represented using interfaces. For example, the weak is-an-extension-of relationship “students are comparable based on their grades” can be represented by implementing the Comparable interface, as follows:

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Examples F Manager is an employee of XYZ limited corporation. (Inheritance) F Manager uses a swipe card to enter XYZ premises. (Association) F Manager has workers who work under him. (Aggregation) F Manager has the responsibility of ensuring that the project is successful. Manager's salary will be judged based on project success. (Composition) 20

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Class Design 1. Identify classes for the system. 2. Describe attributes and methods in each class. 3. Establish relationships among classes. 4. Create classes.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Example 11.1 Borrowing Loans

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Class Design Guidelines F Designing a Single Class. F Using Modifiers public, protected, private F Using Inheritance or Aggregation F Using Interfaces or Abstract Classes

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Designing a Class F A class should describe a single entity or a set of similar operations. A single entity with too many responsibilities can be broken into several classes to separate responsibilities. The String class, StringBuffer class, and StringTokenizer class all deal with strings, for example, but have different responsibilities.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Classes are usually designed for use by many different customers. To make a class useful in a wide range of applications, the class should provide a variety of ways for customization through properties and methods.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Classes are designed for reuse. Users can incorporate classes in many different combinations, orders, and environments. Therefore, you should design a class that imposes no restrictions on what or when the user can do with it, design the properties to ensure that the user can set properties in any order, with any combination of values, and design methods to function independently of their order of occurrence.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Designing a Class, cont. F Choose informative names for classes, data fields, and methods. Always place the data declaration before the constructor, and place constructors before methods. Always provide a constructor and initialize variables to avoid programming errors.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Using Visibility Modifiers F Each class can present two contracts –one for the users of the class and –one for the extenders of the class F Make the fields private and accessor methods public if they are intended for the users of the class. Make the fields or method protected if they are intended for extenders of the class. F The contract for the extenders encompasses the contract for the users. The extended class may increase the visibility of an instance method from protected to public, or change its implementation, but you should never change the implementation in a way that violates that contract.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Using Visibility Modifiers, cont. F A class should use the private modifier to hide its data from direct access by clients. You can use get methods and set methods to provide users with access to the private data, but only to private data you want the user to see or to modify. A class should also hide methods not intended for client use.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Using Inheritance or Aggregation F In general, the difference between inheritance and aggregation is the difference between the is-an- extension-of relationship and the has-a relationship. –For example, an apple is fruit; thus, you would use inheritance to model the relationship between the classes Apple and Fruit. A person has a name; thus, you would use aggregation to model the relationship between the classes Person and Name.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Using Interfaces or Abstract Classes Both interfaces and abstract classes can be used to generalize common features. How do you decide whether to use an interface or a class? In general, a strong is-an-extension-of relationship that clearly describes a parent- child relationship should be modeled using classes.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Using Interfaces or Abstract Classes, cont. F For example, since an orange is a fruit, their relationship should be modeled using class inheritance. A weak is-an- extension-of relationship, also known as an is-kind-of relationship, indicates that an object possesses a certain property. A weak is-an-extension-of relationship can be modeled using interfaces. –For example, A circle or a rectangle is a geometric object, for example, so Circle can be designed as a subclass of GeometricObject. Circles are different and comparable based on their radius, for example, so Circle can implement the Comparable interface.

Liang, Introduction to Java Programming, Sixth Edition, (c) 2007 Pearson Education, Inc. All rights reserved Statechart diagrams, cont.